Instructables user echo_anomie posted a nifty howto for creating a very simple portable laser projector. With a bright enough laser, you can project an image across long outdoor distances and there’s never any need to adjust focus.

When it’s all assembled, you just drop photo slides in the path of the beam and they are projected wherever you have the thing aimed. It’d be interesting to try this with some kind of LCD in place of the slide. This should let you project eerie green videos on skyscrapers around town.